How can I write (draw) notes or just drawings, with a 2d stroke but on the 3d viewport
There are two tools to do what you want: the grease pencil and annotations. 

The grease pencil can be rendered,

https://www.blender.org/features/grease-pencil/

annotations are just for reference.

https://www.youtube.com/watch?v=-IJ7THnd2ZM